Ultra-high pressure mercury lamp

Abstract

An ultra-high pressure mercury lamp in which the tungsten comprising the electrodes is prevented from spraying during starting and operation, and blackening on the inside wall of the arc tube is prevented is achieved in that at least one of the electrodes has an axial part which is made essentially of tungsten and a starting part which is made essentially of tungsten, at least one of the axial part and of the starting part containing an electron emission material with a smaller work function than tungsten such that the value of S/A in mm 2 /% by weight is in the range 1≦S/A≦1×10 4 , S in mm 2 designating the surface area of the part of the electrode projecting into the arc tube containing the electron emission material and A in % by weight indicating the concentration of the electron emission material in this part.

1. Ultra-high pressure mercury lamp, comprising:
 an arc tube, 
 a pair of opposed electrodes opposite in the arc tube, and 
 at least 0.15 mg/mm 3  mercury in the arc tube as an emission material, 
 wherein at least one of the electrodes has an axial part which is made essentially of tungsten and a starting aid part which is made essentially of tungsten, 
 wherein at least one of the axial part and the starting aid part comprises an electron emission material with a smaller electron affinity than tungsten such that the value of S/A in mm 2 /% by weight is in the range 1.9×10 1 ≦S/A≦1×10 4 , where S in mm 2  is the area of a region of the electrodes projecting into the arc tube that contains the electron emission material and A in % by weight is the concentration of the electron emission material in said region. 
 
   
   
     2. Ultra-high pressure mercury lamp as claimed in  claim 1 , wherein at least one of the electrodes has a part with a larger diameter. 
   
   
     3. Ultra-high pressure mercury lamp as claimed in  claim 2 , wherein the part with the larger diameter has the electron emission material. 
   
   
     4. Ultra-high pressure mercury lamp as claimed in  claim 1 , wherein only the starting aid part contains the electron emission material. 
   
   
     5. Ultra-high pressure mercury lamp as claimed in  claim 1 , wherein the electron emission material contains at least one of the elements yttrium, lanthanum, cerium, barium, strontium, hafnium and zirconium. 
   
   
     6. Ultra-high pressure mercury lamp as claimed in  claim 1 , wherein the starting aid part is coil-shaped. 
   
   
     7. Ultra-high pressure mercury lamp as claimed in  claim 2 , wherein only the starting aid part contains the electron emission material. 
   
   
     8. Ultra-high pressure mercury lamp as claimed in  claim 7 , wherein the electron emission material contains at least one of the elements yttrium, lanthanum, cerium, barium, strontium, hafnium and zirconium. 
   
   
     9. Ultra-high pressure mercury lamp as claimed in  claim 8 , wherein the starting aid part is coil-shaped. 
   
   
     10. Ultra-high pressure mercury lamp as claimed in  claim 3 , wherein the electron emission material contains at least one of the elements yttrium, lanthanum, cerium, barium, strontium, hafnium and zirconium.